The present invention generally relates to fabrication of semiconductor devices and more particularly to a fabrication process of a semiconductor device including a grinding step applied to a rear surface of a semiconductor substrate while protecting a front side thereof by an adhesive medium.
In the fabrication process of semiconductor devices such as a semiconductor integrated circuit on a semiconductor wafer, it is commonly practiced to divide the semiconductor wafer into individual chips by a dicing process.
Before applying such a dicing process, it is generally necessary to grind a rear surface of the wafer opposite to a front surface where the semiconductor devices are formed, such that the wafer has a predetermined thickness. For example, such a grinding process is used to reduce the thickness of a wafer of six-inch diameter from 720 .mu.m to 500 .mu.m or less according to the specification of the semiconductor device.
In order to prevent damaging of the semiconductor wafer such as cracking at the time of the grinding, it is commonly practiced to protect the front surface of the wafer, where the semiconductor devices are formed as noted already, by an adhesive medium such as an adhesive tape. Generally, the adhesive tape used for such a purpose carries, on a tape base, an adhesive layer of an acrylic resin with a thickness of 30-40 .mu.m. The tape base, in turn, is formed of a polymer such as polyolefin or polyethylene and generally has a thickness of 100-150 .mu.m. Further, a polymer material such as polyvinyl chloride is also used for the tape base.
After the grinding process, the adhesive tape is removed. In order to facilitate the removal of the tape without leaving any adhesive material on the wafer, the adhesive layer used for such a purpose is generally added with a surfactant. By doing so, the remaining adhesives on the substrate can be removed completely by cleaning the wafer in a purified water or in an organic solvent, if necessary.
Further, there is a proposal to use an adhesive tape that carries an UV-cure type adhesive on the tape base for the purpose of the protection of the wafer during the grinding process. When using such a UV-cure type tape, an ultraviolet radiation is applied to the wafer covered by the tape before removing the tape from the wafer for facilitating the removal of the tape. As the adhesive is cured as a result of the ultraviolet radiation, the adhesion of the tape to the wafer is reduced substantially and the removal of the tape is achieved easily, without applying excessive stress to the wafer.
However, the use of such a conventional tape that contains a surfactant in the adhesive is disadvantageous in that a cleaning process is necessary and inevitable after the removal of the tape, wherein the cleaning process may have to be continued for at least 15-60 minutes. In relation to the removal of the adhesive, it should be noted that the composition of the adhesives used in such a tape changes substantially lot by lot, and the adhesive of the tape tends to establish a very intimate adhesion with the wafer surface particularly when materials such as amorphous carbon, nitrides or amorphous polyimides are remaining on the wafer surface. Such a strong adhesion suggests that there is a crosslink reaction between the adhesive and the residual materials on the wafer. A similar problem occurs also in the tape carrying the UV-cure type adhesives.
Thus, it has been necessary to carry out the cleaning of the substrate surface for a prolonged time duration. As the surface of the wafer is generally protected by a film such as a PSG (phosphosilicate glass), a polyimide or SiN, the tape adhesive remaining on the wafer surface after the tape removal, raises a serious problem.
In order to avoid the problem and to eliminate the cleaning process, it is possible to apply an ozone ashing process for a limited time interval against the wafer surface after the removal of the tape, such that the remaining organic materials are oxidized. However, such an ozone ashing process requires a huge investment for the facility and the cost of the produced semiconductor device is increased inevitably. It is also possible to apply a post-treatment process by using an organic solvent such as isopropyl alcohol for removing the remaining adhesives. However, such a post-treatment process inevitably lowers the throughput of production of the semiconductor device.
Further, the use of a UV-cure tape in the grinding process may cause a problem in the fabrication of a semiconductor memory device that includes a so-called floating gate, such as a flash memory or EEPROM. More specifically, the initial data written into the floating gate of the device may be erased or modified as a result of the ultraviolet radiation.
For example, the data written into the floating gate of a flash memory is destroyed when an ultraviolet radiation is applied to the substrate with a wavelength of 253.7 nm and a radiation flux or dose of 1500 mW/cm.sup.2. When a ultraviolet radiation of a 365 nm wavelength is used, the destruction of the data occurs when the radiation dose is 400 mW/cm.sup.2.
The foregoing problem of destruction of the data as a result of the ultraviolet radiation may be avoided by covering the substrate surface by a polyimide film. However, such a process again requires a huge investment and increases the cost of the semiconductor device. Because of this reason, the use of the UV-cure protective tape has not been made as a matter of common practice for fabricating low cost semiconductor devices.